The present invention relates to dewaxing of petroleum products and other heavy hydrocarbon mixtures. It also relates to similar processes for deoiling the waxes that are found in combination with heavy hydrocarbon mixtures. The present invention also relates to wax fractionation and the production of low pour point oils. It will be understood that when the term dewaxing is used herein it will also include other similar processes such as deoiling. Wax as used in the present description will include all compounds or mixtures to which the term wax is applied, both natural and synthetic, and also will include in general saturated hydrocarbon chain link compounds.
Crude petroleum and partially refined petroleum commonly contain waxes (usually paraffin waxes). Such waxes crystallize at low temperatures, and this is particularly notable with high molecular weight n-paraffins, certain iso-paraffins, and cycloparaffins.
When the petroleum is being refined for use as lubricating oil, the presence of these materials which crystallize within a range of temperatures for which the lubricating oil is intended is very deleterious. Such materials are therefore commonly removed in the refining process and this subprocess is referred to as dewaxing.
There is great variety in the processes used for dewaxing as it cannot be carried out as a normal consequence of the conventional fractional distillation process. The oldest and simplest form of dewaxing is chilling of the crude lubricating oil to about the desired pour point temperature causing crystallization of most of the wax components, after which they are physically removed by filtration or the like. This process is largely of historical interest because of its high cost and unsuitability for processing heavy oils.
The straight chilling process for dewaxing was improved by inclusion of an initial step of adding a relatively large proportion of solvent or diluent to the oil prior to the chilling process. Early types of diluents used in this solvent dewaxing process were heavy naphtha or gas oil. In recent years a great variety of solvents have been employed in conjunction with the chilling step to endeavor to increase efficiency and otherwise improve results.
It was found that somewhat better solvent-chilling dewaxing results were achieved with a mixture of two solvents and thus various mixtures of two different solvents have been employed as dewaxing solvents. Perhaps the most frequently used dewaxing solvent mixture has been a mixture of methyl ethyl ketone (MEK), toluene, and benzene. A common dewaxing solvent mixture may contain 25% to 50% of MEK, 40% to 60% of benzene, and 12% to 25% of toluene. Sometimes ketones of higher molecular weight have been used in place of MEK. This permits one to obtain a higher solvent power for heavy oil. In some solvent-chilling dewax operations the high crystallization temperature (about 5.degree. C.) of benzene has caused toluene to be substituted for the benzene so that the diluent is essentially a mixture of MEK and toluene.
A common solvent-chilling dewax process may proceed as follows. The solvent may be an MEK/benzene or an MEK/toluene combination. After the solvent is added to the oil charge to form a mixture, the mixture is normally heated slightly to insure complete solution of wax components. The mixture is then chilled to the required filtration temperature, usually on the order of -20.degree. C. utilizing a conventional refrigeration process. Refrigeration is typically carried out by pipe-in-pipe type heat exchangers (scrape-surface heat exchangers) with the solvent and waxy oil in the inside pipe and a refrigerant such as propane or sulfur dioxide in the annular space between the two pipes. The surface of the inner pipe must be kept free of wax by scraper blades to maintain adequate heat transfer. The wax is removed by filtration under vacuum in conventional rotary filters in a well known manner.
There are other processes for solvent-chilling dewaxing, such as propane dewaxing in which a single effective constituent is present in the solvent. Propane dewaxing has certain advantages in that it may be a follow-on to propane deasphalting, thereby eliminating a propane-oil separation step between the stages of the process. A disadvantage of propane dewaxing is that the required dewaxing temperatures are generally lower.
In light of the foregoing, there is a need for an improved, simplified and economical petroleum-wax separation process which provides for the effective dewaxing or deoiling of waxy feedstocks.